1. Field of the Invention
This invention resides in the technology of engine design for rocket propulsion systems.
2. Description of the Prior Art
Rocket-powered launch vehicles require high thrust at takeoff due to the need to lift the vehicle from a standing position and also due to the vehicle weight which is greatest at takeoff because of the unburned fuel. For vehicles launched from the earth's surface, takeoff typically occurs at sea level while the vehicle performs its primary mission at high altitude where the external pressure is subatmospheric and often at high vacuum. The vehicle thus needs an engine that provides both a high thrust at sea level to reach high altitude and a high specific impulse Isp (the ratio of the thrust to the weight of fuel consumed in a unit of time) at high altitude where the rocket performs its mission. These are conflicting needs since high Isp is most readily achieved by a nozzle with a high area ratio, i.e., a high ratio of the cross-sectional area at the nozzle exit to the cross-sectional area at the throat, and yet at sea level, the expansion caused by a high area ratio produces an internal pressure at the nozzle wall near the nozzle exit that is below the external (atmospheric) pressure, resulting in a reverse pressure differential and thereby a negative thrust component, i.e., one whose direction is opposite to the forward direction of the vehicle.
The prior art offers several ways of eliminating the negative thrust component at low altitudes without compromising the thrust in the high vacuum environment of the higher altitudes. One is the use of a nozzle of variable exit area, in which the exit area is reduced for launch and then gradually increased during ascent. The variation is achieved by constructing the nozzle with the capability of adjustments to the contour, area ratio, and length of the nozzle as the vehicle altitude increases. These features add considerably to the complexity and weight of the engine construction, however, and they are less than fully successful since the nozzle in most cases continues to produce less thrust at sea level than at vacuum. Other methods have included the use of combination-type engines using different fuels at different stages. One such combination is the dual-fuel-dual-expander engine concept described by Beichel, R., in U.S. Pat. No. 4,220,001 (issued Sep. 2, 1980). The Beichel engine entails a nozzle of complex design that incorporates two thrust chambers. Among the various alternatives to the Beichel engine are engines in which secondary combustion gas is introduced near the wall of the divergent section of the nozzle, as described by Bulman, M., in U.S. Pat. No. 6,568,171 (issued May 27, 2003); engines with expansion-deflection and plug nozzles that incorporate secondary injectors, as described by Bulman, M., et al., in U.S. Patent Application Publication No. US 2007/0056261 A1 (published Mar. 15, 2007); and an engine with a throttling injector to achieve a variable area ratio, as described by Wagner, W. R., in U.S. Pat. No. 4,644,475 (issued Feb. 24, 1987). In the Wagner engine, the injector is at the forward end of a divergent combustor, and the purpose of the throttling is to move the site within the combustor at which sonic conditions are achieved, from locations close to the forward end to sites approaching the nozzle throat. Controlling this from the forward end requires an injector that produces much finer and better mixed propellants at low thrust than at high thrust. Although this is theoretically achievable, it requires additional control of the combustion process such as multiple pintle injectors which leads to more complex and heavy engines.
Of further possible relevance to this invention are vehicles that are designed to undergo ascent, descent, or both in high-vacuum environments such as the surface of the moon. Such a vehicle requires very deep throttling when approaching a landing surface in a high-vacuum environment, in addition to the ability to vary its thrust upon takeoff depending on whether takeoff occurs in a high-pressure environment such as the surface of the earth or a low-pressure environment such as the surface of the moon.